In recent years, instead of using aluminum or aluminum alloys as a material for forming interconnection circuits on a substrate such as a semiconductor wafer, there is an eminent movement towards using copper (Cu) which has a low electric resistance and high electromigration resistance. Copper interconnects are generally formed by filling copper into fine recesses formed in a surface of a substrate. There are known various techniques for forming such copper interconnects, including CVD, sputtering, and plating. According to any such technique, a copper film is formed on the substantially entire surface of a substrate, followed by removal of unnecessary copper by chemical mechanical polishing (CMP).
FIGS. 13A through 13C illustrate, in sequence of process steps, an example of forming such a substrate W having copper interconnects. As shown in FIG. 13A, an insulating film 2, such as a silicon oxide film of SiO2 or a film of low-k material, is deposited on a conductive layer 1a in which electronic devices are formed, which is formed on a semiconductor base 1. A contact hole 3 and a trench 4 for interconnects are formed in the insulating film 2 by the lithography and etching technique. Thereafter, a barrier layer 5 of TaN or the like is formed on the entire surface, and a seed layer 7 as an electric supply layer for electroplating is formed on the barrier layer 5.
Then, as shown in FIG. 13B, copper plating is performed onto the surface of the substrate W to fill the contact hole 3 and the trench 4 with copper and, at the same time, deposit a copper film 6 on the insulating film 2. Thereafter, the copper film 6 and the barrier layer 5 on the insulating film 2 is removed by chemical mechanical polishing (CMP) so as to make the surface of the copper film 6 filled in the contact hole 3 and the trench 4 for interconnects and the surface of the insulating film 2 lie substantially on the same plane. An interconnection composed of the copper film 6 as shown in FIG. 13C is thus formed.
In this case, the barrier layer 5 is formed so as to cover the substantially entire surface of the insulating film 2, and the seed layer 7 is also formed so as to cover the substantially entire surface of the barrier layer 5. Thus, in some cases, a copper film that is the seed layer 7 resides on a bevel (outer peripheral portion) of the substrate W, or copper is deposited on an edge (outer peripheral portion), which is inward of the bevel of the substrate W, and remains unpolished. Copper can easily be diffused into the insulating film in a semiconductor fabrication process such as annealing, thus deteriorating the electric insulation of the insulating film, and may cause cross contamination in subsequent processes of delivering, storing and processing the substrate. For these reasons, it is necessary that the remaining deposited copper on the peripheral portion of the substrate should be completely removed. Therefore, it is suggested that conductive material such as copper deposited on or adhering to the peripheral portion of the substrate will be removed by an etching process or the like.
As described above, the impurity contamination in the production of a semiconductor device greatly affects the reliability of the semiconductor device. Accordingly, with respect to a substrate in which a film has been formed e.g. by plating over the entire surface e.g. for the formation of semiconductor interconnects or contacts, the substrate is usually subjected to a process for removing the film on a peripheral portion of the substrate in order to prevent a later contamination of a processing device which would be caused by contact between the film and a substrate transport device. Such a film removal processing has generally been carried out by supplying an etching liquid only to a to-be-removed region of a substrate to effect removal of a film only in the to-be-removed region.
Components in various types of equipment have recently become finer and have required higher accuracy. As sub-micro manufacturing technology has commonly been used, the properties of materials are largely influenced by the processing method. Under these circumstances, in such a conventional machining method that a desired portion in a workpiece is physically destroyed and removed from the surface thereof by a tool, a large number of defects may be produced to deteriorate the properties of the workpiece. Therefore, it becomes important to perform processing without deteriorating the properties of the materials.
Some processing methods, such as chemical polishing, electrolytic processing, and electrolytic polishing, have been developed in order to solve this problem. In contrast with the conventional physical processing, these methods perform removal processing or the like through chemical dissolution reaction. Therefore, these methods do not suffer from defects, such as formation of an altered layer and dislocation, due to plastic deformation, so that processing can be performed without deteriorating the properties of the materials.
When removing a conductive material, such as copper, by e.g. a common etching processing technique conventionally employed, a chemical liquid, selected from a variety of kinds, is used. This requires an adequate post-cleaning and, in addition, imposes a considerable load upon waste liquid treatment. Also in this connection, it is to be pointed out that though a low-k material, which has a low dielectric constant, is expected to be predominantly used in the future as a material for the insulating film of a semiconductor substrate, the low-k material has a low mechanical strength and therefore is hard to endure the stress applied during CMP processing.
Further, with such an etching processing (film-removing processing), control of an etching width and of an edge configuration cannot be made with ease. In addition, with the progress towards multi-layered interconnects, there is the problem of an increased number of process steps becoming necessary.
A method has been reported which performs CMP processing simultaneously with plating, viz. chemical mechanical electrolytic polishing. According to this method, the mechanical processing is carried out to the growing surface of a plating film, causing the problem of denaturing of the resulting film.
In the case of the above-mentioned conventional electrolytic processing or electrolytic polishing, the process proceeds through an electrochemical interaction between a workpiece and an electrolytic solution (aqueous solution of NaCl, NaNO3, HF, HCl, HNO3, NaOH, etc.). Since an electrolytic solution containing such an electrolyte must be used, contamination of a workpiece with the electrolyte cannot be avoided.